Because of its relatively low cost, linear tape is commonly used as a medium for storing large amounts of digital data for archival purposes. For example, disk-based memory is often archived on linear data storage tape.
Data is formatted on linear tapes in a plurality of tracks that extend longitudinally along the tape. A tape head is moveable laterally across the tape to read or write different tracks. In many cases, multiple tracks can be written or read at the same time by using a tape head with multiple read/write elements.
When reading or writing a linear data storage tape, accurate lateral positioning of the tape head is very important. To achieve such accuracy, servo bands are prewritten to the tape. The servo bands are detected by the tape head during reading and writing to determine the exact lateral position of the tape head relative to the linear tape.
To illustrate the use of servo bands, FIG. 1 shows a segment of a linear tape 10 that extends in a longitudinal direction x, and has a lateral dimension y. The tape 10 includes a plurality of servo bands 12. In the simplified example of FIG. 1, there are three servo bands. The servo bands are written to the tape during a preparatory formatting process known as "servo writing", prior to actual use of the tape for data storage. The servo bands are spaced laterally from each other by a specified distance.
Data tracks 14 are located between the servo bands 12. The lateral positions of the data tracks 14 are specified relative to the servo bands 12.
When reading or writing tape 10, a tape head senses the servo bands 12 with servo read elements and positions itself precisely relative to the servo bands. Within the tape head, data read/write elements are spaced relative to the servo read elements so that the data read/write elements will be positioned over data tracks 14 when the servo read elements are positioned accurately over the corresponding servo bands 12.
In an actual embodiment, a linear tape might have more than three servo bands. One type of tape head is configured to span two adjacent servo bands at any given time and to read or write only the data tracks between those servo bands. This type of tape head is repositioned to span two different servo bands when reading or writing other data tracks.
There are different ways to derive lateral position information from a servo band. One common way is to divide a servo band into two or more tracks, which are recorded with different information (such as distinct frequencies or bursts occurring at distinct times). A single servo head straddles the boundary between the tracks, and position information is obtained by comparing the amplitude or phase responses of the signals generated from the respective tracks.
A different approach has been described in Albrecht, et al., Time-Based, Track-Following Servos for Linear Tape Drives, Data Storage Magazine, 1997 (p. 41), which is hereby incorporated by reference. This approach uses a timing-based servo in which a narrow servo head reads a continuously variable servo band.
FIG. 2 shows an example of a continuously variable, timing-based servo pattern, along with a signal generated by a servo read element positioned over the servo pattern. The pattern consists of alternating magnetic transitions at two different azimuthal slopes. Relative timing of pulses generated by the read element depends on the lateral position of the head.
More specifically, the servo band illustrated in FIG. 2 has a series of magnetic transitions 20 and 22, referred to as "stripes", that are recorded on the tape with alternate azimuthal slopes. The stripes 20 have positive slope, while the intervening stripes 22 have negative slopes.
FIG. 2 shows the path and width of the servo head, indicated by reference numeral 24. The servo head reads a lateral width that is significantly less than the full lateral width of the stripes themselves. The signal generated by the servo head is represented by trace 26, illustrated directly below the illustrated magnetic transition stripes. When the servo head encounters a stripe, it generates a positive pulse. When the servo head leaves the stripe, it generates a negative pulse.
Lateral position information is derived by comparing the distances between pulses. For example, a first distance A can be defined as the distance from a positive stripe to the next negative stripe, while a second distance B can be defined as the distance from a negative stripe to the next positive stripe. When the servo head is centered over the servo band, distance A will be equal to distance B, causing consecutive pulses to occur at equal intervals. When the servo head is not centered over the servo band, distance A does not equal distance B, resulting in alternating long and short pulse intervals.
In actual implementation, alternating "bursts" of stripes are used. A burst is defined as one or more individual magnetic transition stripes.
FIG. 3 shows an example of a servo band layout utilizing alternating bursts of magnetic transition stripes. Each burst has an opposite azimuthal slope from the previous burst. The servo pattern includes repeating frames. Each frame has a first subframe A and a second subframe B. Each subframe has a pair of bursts, with the bursts of each frame having different azimuthal slopes. Subframe A has a first burst 38 with five equally spaced stripes having a positive azimuthal slope. Subframe A has a second burst 40 with five equally spaced stripes having negative azimuthal slopes. Subframe B has similar bursts 42 and 44, except each of these bursts has only four stripes rather than five.
During reading and writing, the tape is moved passed the tape head in the longitudinal direction at a specified velocity. This velocity is typically tracked by a sensor on the mechanical drive system responsible for moving the tape passed the tape head. The specified velocity is optimized to perfect conditions in which the tape head remains stationary and the tape moves precisely along a longitudinal path perpendicular to the lateral dimension of the tape. However, it is common for the tape head to move in the lateral direction across the tape as the tape is moving underneath in the longitudinal. Furthermore, the tape itself may not always pass perfectly along the longitudinal path underneath the tape head. Conventional techniques for determining linear tape speed do not account for this lateral component. As a result, the actual velocity of the tape relative to the tape head may not be the desired optimum velocity being applied by the tape drive system.
Accordingly, there is a need for a technique that determines the true linear speed of the tape beneath the tape head, independent of the head's lateral movement across the tape.